Hi,
I think it might be valuable to have a web forum. Not as a different interface to the same community, but as a different community.
Perhaps the sort of people that prefer mailing lists and the sort of people that prefer web forums are different after all, and a community that grows up around a forum could serve different needs and exhibit different characteristics to that of the mailing list.
It would not be fragmentation, it would be growth, and I imagine there would be a degree of overlap.
For me the choice of mailing list or web forum depends on the topic, and how I want to use it, I think for me, when it comes to feeling part of a Haskell community a web forum may suit better than the mailing list, so I would like to try it.
For me mailing lists are great for focussed technical questions and discussions, but less useful for building a community. A web forum tends to be less sterile and allows more personality to show through and I guess it allows a bit more of the culture surrounding the core tech to flourish, as members begin to form images of each other. Definitely not for many of you I am sure, but I think if the goal is to nourish a culture and encourage a true community rather than just technical Q&A, then a web forum would be an intriguing option.
But a different interface to the same community doesn't make much sense. And let us try and find a Haskell based software solution for it, I thought phpBB was a joke in the OP.